The SLE had 133K on it when the fuel economy suddenly dropped from 24 to 18 mpg within two tanks of gas. The subject line - "Fuel economy restored" just doesn't grab you, but would have been more accurate. Idon't know if the previous owner had ever changed it. The old one was an AC/Delco. I think it is more common for them to go bad slowly. If your mpg is still good, the O2 is probably OK. My '95 is stock except for a gutted air box. Considering that my combined mpg is 24, I'm thinkin' on a cool day with no AC, no wind, flat roads, all trip miles, and a gentle touch, I might be able to come close to 30mpg.

we have been preeching this one for a long time now... O2 sensors do go a long ways in terms of gas milage... Especially sudden drops...

Yup, you are pretty much dead on... after my replacement I did a 23.3 in town...

Bill I'll bet at 70 mph for an extended period of time, you'll do 30-32 easily ;)

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No idea when my 97 had one..but yes.. I was able to net 33.5 on an extended trip.

Pretty good mileage on trips considering GM only told us mid 20's..

just tossin my $.02 cents in here, i noticed a heavy decline in mpg after replacing my upper intake after it went....i would say that the massive amount of coolant i burned off messed up the O2 sensor, i puffed white smoke for a good 5-10 minutes.. so i would say definetely check and/or replace the sensor if youve had intake failure..anyone else agree? disagree?

Agree it's worth pulling and looking at.. if ugly..replace

Very true... This is how my first sensor died... It did 23-24 in town and 27 mixed... After that it was very very low... ;)

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They get lazy like we do
 

What happens as the O2 sensor gets old from use and many hot cold cycles they start to respond slower to the oxygen level in the exhaust. That is why they still will read ok with scan tool but when they are replaced u will see mpg go up. The sensor in front of the cat is the most important one regarding mpg. The rear one senses if catalytic converter is working properly.